fix(external_storage): fix settings save

[skjnldsv]

• Returning boolean on a controller just output said boolean as plain text in the reponse
• Encoding JSON before sending the request with axios will result in double encoding and failure to be read by the backend
• Added error handling from the backend too

Fix#49829
Followup from #50910

Regression from

• fix: add PasswordConfirmationRequired to the external storages mentioned in review #48788
• feat: Use inline password confirmation in external storage settings #49493

[skjnldsv]

/backport to stable31

[skjnldsv]

/backport to stable30

[skjnldsv]

/backport to stable29

[susnux]

Makes sense

[AndyScherzinger]

🙏

[skjnldsv]

/compile

[nsemov]

I'm running 31.0.0 and noticed that when saving global credentials the browser says it submits payload:
{"uid":"","user":"asdasd","password":"asdasdasd"}

But by the time saveGlobalCredentials is called in AjaxController, $uid is NULL rather than an empty string, so the $uid === '' comparison fails and permission to set the credentials is denied.

Unfortunately I'm too basic to see if all these changes that were implemented will correctly pass an empty string to the saveGlobalCredentials rather than a NULL.

[Edit]
Actually, all 3 parameters passed to saveGlobalCredentials are NULL ... :-(

[skjnldsv]

/compile amend /

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31
# Create the new backport branch
git checkout -b backport/51320/stable31
# Cherry pick the change from the commit sha1 of the change against the default branch# This might cause conflicts, resolve them
git cherry-pick 07264dff 2f4b4ce4
# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/51320/stable31

Error: Failed to create pull request: Validation Failed: {"resource":"PullRequest","field":"head","code":"invalid"}

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[skjnldsv]

/backport 07264df to stable31

[skjnldsv]

Error: Failed to create pull request: Validation Failed: {"resource":"PullRequest","field":"head","code":"invalid"}

That's a new one 🤔